7-year-old spayed female Yorkshire Terrier

MRI system: 1.5 T magnet (Magnetom EspreeTM, Siemens Medical Technologies, Malvern, PA). Sequences (Cervical and Cranial Thoracic Spine):

• Dorsal short tau inversion recovery (STIR) images (TR 3100ms, TE 40ms, TI 160ms, 3 mm slice thickness)
• Sagittal T2-W images (TR 2370ms, TE 107ms, 2.5 mm slice thickness)
• Sagittal T1-W images (TR 320ms, TE 13ms, 2.5 mm slice thickness)
• Sagittal short tau inversion recovery (STIR) images (TR 3100ms, TE 43ms, TI 160ms, 2.5 mm slice thickness)
• Sagittal half-Fourier-acquisition single-shot turbo spin-echo (HASTE) images (TR 8000ms, TE 257ms, 0.8 mm slice thickness)
• Transverse T2-W images (TR 2560ms, TE 93ms, 2.5 mm slice thickness)
• Transverse T1-W images (TR 360ms, TE 17ms, 2.5 mm slice thickness)
• Post contrast T1-W images with fat suppression in transverse and sagittal plane obtained after intravenous injection of 0.2 ml/kg (0.1 μmol/kg) gadopentetate dimeglumine (Magnevist®)

There is an identification microchip within the soft tissues dorsal to the caudal cervical spinal cord resulting in a large magnetic susceptibility artifact that limits evaluation in this region.

The dorsal arch of C1 is displaced cranially into the foramen magnum and is overlapping with the caudoventral aspect of the occipital bone. The dorsal arch of C1 is causing moderate compression of the caudoventral aspect of the cerebellum and marked narrowing of the cerebellomedullary cistern. There is dorsal angulation of the dens, and the dens is displaced dorsally from the ventral aspect of the spinal canal and body of C1 causing moderate spinal cord compression. There is mild kinking of the junction of the brain stem and spinal cord at the level of the atlanto-occipital and atlanto-axial joints. The spinal cord resumes a normal diameter at the mid body of C2. There is a focal T2-W hyperintensity within the spinal cord parenchyma adjacent to the dens. There is mild dilation of the central canal and a linear T2-W hyperintensity within the dorsal spinal cord parenchyma extending from the level of C2-C3 caudally to approximately the level of the C5 vertebra. The portion of the brain parenchyma included in the imaging field of view is normal; however, there is mild dilation of the ventricles of the brain. There is no abnormal contrast enhancement observed.

1. Atlanto-axial subluxation, dorsal angulation of the dens and atlanto-occipital overlapping resulting in moderate compression of the cervical spinal cord by the dorsally displaced dens and compression of the caudoventral aspect of the cerebellum by the dorsal arch of C1.
2. Mild syringohydromyelia likely secondary to chronic alteration in CSF flow by the atlanto-occipital overlapping and atlanto-axial subluxation. Mild hydrocephalus may be a breed-associated variant of normal or due to partial occlusion of the CSF outflow tract caused by compression by the atlanto-occipital overlapping.

Discussion

Following MRI a CT examination was performed for better assessment of the osseous structures of the craniocervical junction. In addition to the abnormalities found on MRI, CT revealed occipital malformation with enlargement of the foramen magnum (see sagittal reconstructed CT image). Due to the age of the patient, the chronic history and the invasive nature of surgical intervention the dog’s owners elected conservative treatment (neck brace and pain management).Atlanto-occipital overlapping is a fairly recently recognized abnormality and is part of a group of complex craniocervical junction abnormalities seen in small breed dogs (chiari like malformation, atlanto-occipital instability, atlantoaxial instability, occipitoatlantoaxial malformation, atlantooccipital overlapping and dens abnormality) (1). Like in this dog, patients are commonly afflicted with more than one abnormality, underlining the need for advanced imaging (MRI +/- CT) to establish a definitive diagnosis and plan for surgical intervention (2,3).

References

1. Marino DJ, Loughin CA, Dewey CW, Marino LJ, Sackman JJ, Lesser ML, Akerman MB. Morphometric features of the craniocervical junction region in dogs with suspected Chiari-like malformation determined by combined use of magnetic resonance imaging and computed tomography. Am J Vet Res 2012;73(1):105-11.
2. Cerda-Gonzalez S, Dewey CW, Scrivani PV, Kline KL. Imaging features of atlanto-occipital overlapping in dogs. Vet Radiol Ultrasound 2009;50(3):264-8.
3. Dewey CW, Cerda-Gonzalez S, Scrivani PV. Case report –surgical stabilization of a craniocervical junction abnormality with atlanto-occipital overlapping in a dog. Compend Contin Educ Vet 2009;31(10):E1-6.